Project Summary: Angiogenesis ? the formation of new blood capillaries - plays a key role in organ regeneration. Thus, in order to regenerate adult organs, we need to understand the mechanisms of angiogenesis during organ regeneration. It has been reported that adult human lungs have potential to grow after unilateral pneumonectomy (PNX) and that inhibition of angiogenesis impairs post-PNX lung growth in adult mice. The overall goal of this proposal is to characterize the mechanism of angiogenesis during organ regeneration using lung as a model and leverage this knowledge to develop an efficient strategy for organ regeneration. Mechanosensitive transcriptional co- activators, Yes-associated protein (YAP1) and transcriptional co-activator with PDZ-binding motif (TAZ), stimulate angiogenesis and control organ size and regeneration. In the lung, YAP1 activation in alveolar stem cells promotes post-PNX lung growth. However, the role of endothelial YAP1/TAZ in angiogenesis during post- PNX lung growth remains unclear. Our preliminary data demonstrate that knockdown of YAP1 and/or TAZ decreases the expression of angiogenic factor receptor, Tie2, in endothelial cells (ECs) and inhibits EC sprouting. After unilateral PNX, the expression of YAP1/TAZ and Tie2 and vascular density increased in the remaining lung lobes. Compensatory lung growth after PNX was inhibited in VE-cadherin-specific Yap1 and/or Taz knockout (Yap1fl/fl-Cdh5CreERT2, Yap1i?EC, Taz i?EC, or Yap1/Tazi?EC) mice after tamoxifen-induced Yap1/Taz deletion. When we implanted fibrin gel on the control Yap1fl/fl mouse lung, ECs were recruited from host lungs and made vascular networks in the implanted gels, while vascular formation was attenuated in the gel implanted on the Yap1fl/fl-Cdh5CreERT2 mouse lungs. Parenchymal stretch and increases in microvascular perfusion and shear stress after unilateral PNX contribute to post-PNX lung growth. Insertion of silicone prosthesis to replace an excised lobe prevented post-PNX lung growth and decreased YAP1/TAZ expression. Ligation of right cardiac lobe pulmonary artery stimulated lung growth of the remaining non-occluded lobes after left PNX. We hypothesize that angiogenesis is stimulated during compensatory lung growth after PNX through mechanosensitive endothelial YAP1/TAZ signaling. In Aim 1, we will investigate the mechanism by which endothelial YAP1/TAZ control angiogenic signaling during post-PNX lung growth in vitro. In Aim 2, we will determine whether endothelial YAP1/TAZ control blood vessel formation during post-PNX lung growth in the mouse lung. In Aim 3, we will investigate the effects of mechanical environment altered by PNX on blood vessel formation in the mouse lung. Our focus on the new mechanosensitive mechanism of angiogenesis during post-PNX lung growth and the novel mouse lung gel implantation system are highly unique and innovative scientific advances. If this study validates that endothelial YAP1/TAZ stimulate angiogenesis in the mouse lungs after PNX, this work will further our understanding of the mechanism of angiogenesis and lead to the development of new and better strategies for organ regeneration.